The present invention relates generally to the field of data storage, and more particularly to supporting small volumes of data on a grid-scale disk storage system.
In computer storage, a logical unit number, or LUN, is a number used to identify a logical unit, which is a device addressed by the Small Computer System Interface (SCSI) protocol or Storage Area Network (SAN) protocols which encapsulate SCSI, such as Fibre Channel or iSCSI. A LUN may be used with any device which supports read/write operations, such as a tape drive, but is most often used to refer to a logical disk as created on a SAN. The term “LUN” is often also used, and will be used throughout this application, to refer to the logical disk itself.
To provide a practical example, a typical multi-disk drive has multiple physical SCSI ports, each with one SCSI target address assigned. An administrator may format the disk array as a redundant array of independent disks (RAID) and then partition this RAID into several separate storage-volumes. To represent each volume, a SCSI target is configured to provide a logical unit. Each SCSI target may provide multiple logical units and thus represent multiple volumes, but this does not mean that those volumes are concatenated. The computer that accesses a volume on the disk array identifies which volume to read or write with the LUN of the associated logical unit. In another example: a single disk-drive has one physical SCSI port. It usually provides just a single target, which in turn usually provides just a single logical unit whose LUN is zero. This logical unit represents the entire storage of the disk drive.
In a grid-scale storage system like IBM XIV Storage System (“XIV”), each LUN is spread across the entire system. XIV uses 180 disks and each LUN occupies at the minimum storage space, which is approximately 17 GB, as at least one 1 MB partition is reserved in each of the 16,411 slices. It should be appreciated that 18 disks is the minimum number of disks for XIV code running on IBM Spectrum Accelerate. It should also be appreciated that 78 disks is the minimum number of disks for XIV code running on XIV hardware.